This invention relates to a fluorescent lamp. More particularly the invention relates to a fluorescent lamp having a phosphor layer formed by using water-insoluble glass as adhesive material. The invention also pertains to a phosphor coating composition in the form of a slurry used for producing such fluorescent lamp.
For forming a phosphor layer on the inner wall of the glass tube in the conventional fluorescent lamps, there have been generally employed the techniques comprising suspending a phosphor and a low-melting-point glass powder in a solution prepared by dissolving a binder such as nitrocellulose in butyl acetate, coating the interior surface of the glass tube with this suspension, and then drying and baking the coating. If no adhesive material such as low-melting-point glass is added in said composition, the formed phosphor layer does not adhere fast to the glass tube and is prone to exfoliation. Particularly in the case of circular lamp, such phosphor layer tends to exfoliate in the step where the glass tube is shaped circularly.
Use of an organic solvent such as butyl acetate as in said composition is undesirable because of flammability of such organic solvent, its harmfulness to the human body, economy and other reasons. However, if a water-soluble binder solution is used, said low-melting-point glass might be dissolved or decomposed in water to give a baneful effect to the lamp characteristics of the produced fluorescent lamp. A variety of low-melting-point glass compositions have been proposed, but for example the prevalently used CaO-BaO-B.sub.2 O.sub.3 glass is decomposed in water to elute B.sub.2 O.sub.3 and hence unusable for the purpose said above.